On-site Evaluation of Concrete Strength is a main challenge in the condition assessment of existing infrastructure, or the quality control of new construction. Owners, maintenance managers of such existing concrete structures normally prefer non-destructive and non-intrusive methods to avoid further damage to an already struggling structure. In construction projects, switching to non-destructive means less intervention, shorter down-times, and saving money. However, all parties agree that the strength of concrete is a critical parameter. In this article, we will review the potential options and practical solutions for on-site evaluation of concrete strength.
On-site Evaluation of Concrete Strength
Concrete strength (compressive strength) is by far the most important property of concrete. It represents the mechanical characteristics of concrete; The 28 days compressive strength of concrete cylinders or cube samples has widely been accepted as the minimum specified concrete strength in most design codes (ACI 318-14, CSA A23.3-14). Concrete Strength is also considered a key factor in obtaining desired Durability Performance.
Evaluation of Concrete Strength is an important task:
- Existing Structures: Concrete Strength is of special interest for engineers involved in the Structural Condition Assessment of concrete structures. It is used to assess the mechanical characteristics and durability performance of concrete.
- New Construction: Concrete Strength is usually monitored during the construction process. Construction engineers, project managers, and Quality Control and Quality Assurance auditors depend on the compressive strength test results. When compression test on concrete cylinders yield low breaks, engineers require reliable tools to assess the actual strength of concrete.
Nondestructive testing (NDT) offer an interesting approach to evaluating the compressive strength of concrete. NDT methods provide access to material properties while remaining rapid and of moderate cost (Breysse, 2012). The following article will take a quick look at some of the major non-destructive testing solutions for on-site evaluation of concrete strength. In the first part, we will present and discuss NDT methods for evaluating concrete strength in existing structures. In the second part, we will present and review the NDT methods for evaluation of early age strength of concrete.
Part I - Existing Structures
1- Compression Test On Concrete Cores
Extracting concrete samples (Read More: Challenges of Concrete Coring) and testing for compressive strength is often considered the most cost-effective and most reliable solution. In fact, many codes and guidelines consider this the only approved method for evaluating concrete strength. In this case, concrete core is taken from the existing structure.
The core needs cutting (sawing) and surface preparation. The core is then tested for compressive strength. However, the reality is far from this. There are certain questions that needs to be answered: Where to take concrete cores from? How to handle cores properly (maintain moisture, safe mobilization)? How many cores will yield reliable information?
- This is the most reliable method to estimate the compressive strength. The method is relatively fast.
- It is destructive. Not only it damages concrete integrity, it might affect reinforcing bars in RC structures. Rebar locating tools, such as Ground Penetrating Radar – GPR are needed to avoid this problem.
- Selecting test locations can be difficult. Selecting the best location of cores is relatively subjective.
- The locations of cores needs to be repaired.
- Coring is not an option for owners of important structures, especially when there are concerns about further damaging the structure.
2- The Pull-Out Test
The concept behind Pull-Out Test is that the tensile force required to pull a metal disk, together with a layer of concrete, from the surface to which it is attached, is related to the compressive strength of the concrete.The pull out test is normally used for early diagnosis of strength problems. However, it can be used to evaluate the strength of concrete in existing structures. Pull out testing involves attaching a small piece of equipment to the exterior bolt, nut, screw or fixing. This is then pulled to the designated stress load level to determine how strong and secure the fixing is.
- Relatively easy to use
- If relationship to strength is established, the method can deliver robust test results.
- Pull-Out test often involves crushing and damaging concrete
3- Rebound Hammer For Concrete Strength
The Rebound Number of Hardened Concrete (see ASTM C805) can be used to assess:
- In-place uniformity of concrete,
- to delineate variations in concrete quality throughout a structure, and
- to estimate in-place strength if a correlation is developed (Read More)
The Rebound Hammer works based on the rebound principle, and consist of measuring the rebound of a spring driven hammer mass after its impact with concrete. New versions of the test have been commercialized and are used to help engineers and inspector with a wider range of material properties.
Due to its simplicity and low cost the rebound hammer is the
most widely used nondestructive test for concrete. It is frequently used-although by mistake- as a tool to assess the strength of concrete. Malhotra (2004) argues that “there is little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer. However, within limits, empirical correlations have been established between strength properties and the rebound number. ACI 228.1R describes a standard procedure to calibrate test results for every specific project, and use project-specific correlation to assess the strength. This will minimize the number of intrusive tests.
- It is easy to use for most field applications.
- The test can be used to study the uniformity of concrete
- surface condition, presence of rebar, presence of sub-surface voids can affect the test results
4- Ultrasonic Pulse Velocity
Ultrasonic Pulse Velocity (UPV) is an effective method for quality control of concrete materials, and detecting damages in structural components.
The UPV methods have been traditionally used for the quality control of materials, mostly homogeneous materials such as metals and welded connections. With the recent advancement in transducer technology, the test has been widely accepted in testing concrete materials. The test procedure has been standardized as “Standard Test Method for Pulse Velocity through Concrete” (ASTM C 597, 2016).
The concept behind the technology is measuring the travel time of acoustic waves in a medium, and correlating them to the elastic properties and density of the material. Travel time of ultrasonic waves reflects internal condition of test area. Some researchers have tried to develop a relationship between the strength and wave speed.
- UPV can be used to detect other sub-surface deficiencies
- The method is affected by presence of rebar, voids, and cracks.
- There is no enough results for assessing the reliability of the method in the field.
5- Combined NDT Methods
As we discussed above, Rebound Hammer and Ultrasonic Pulse Velocity are the most widely used NDT methods for evaluating concrete strength in existing structures (Malhotra, 2004)
Combined methods involves a combination of NDT methods for predicting the on-site strength of concrete. The combination of UPV and Rebound hammer has been studied by several researcher. Breysse, 2012 have conducted a comprehensive literature review on the combined methods.
The improvement of the accuracy of the strength prediction according is achieved by the use of correction factors taking into account the influence of cement type, cement content, petrologic aggregate type, fine aggregate fraction, and aggregate maximum size. The accuracy of the combination of rebound hammer and ultrasonic pulse velocity results in improved accuracy in estimating the compressive strength of concrete (Hannachi and Guetteche, 2012).
It is very important to consider that the accuracy of each and every relationship depends on the calibration and correlation that is made with destructive tests (core samples). While combined methods still rely on intrusive tests to yield accurate outcome, they have a huge potential to reduce the number of destructive tests on a job site.
Part 2 - New Construction
1- The Maturity Method
The maturity method is a technique to account for the combined effects of time and temperature on the strength development of concrete.” (Carino and Lew, 2001). Maturity method provides a simple approach for evaluating the strength of cement-based materials in real-time, i.e. during construction. The test procedure has been standardized in the ASTM C1074 – 19
Maturity method uses the history of temperature variation in concrete elements. Termocouples (wired or wireless) are embedded within concrete, and the temperature variation of concrete during the curing process is monitored in real time.
The Maturity Index is used to correlate test results from Maturity test to compressive strength obtained from cylinder samples cured in laboratory condition. The relationship can be used to monitor strength development in fresh and early age concrete.